This invention relates to thick film circuits provided with fuses, and more particularly to such circuits provided with heat activated solder fuses for protecting the associated circuits.
It is established practice to provide thick film circuits in which electrically conducting tracks disposed on the surface of an electrically insulating substrate incorporate a conductive link comprising or including solder which, when a fault condition arises, melts to cause the associated circuit to be broken.
In one such known arrangement, a spring fuse under compression is soldered between terminals ends of electrically conducting tracks whereby, on melting of the solder, the fuse springs apart and breaks the circuit.
Such fuses are relatively complex and expensive to manufacture, and can be difficult and time-consuming to install.
In another known arrangement, the fuse comprises solder bridging the terminals ends of the electrically conducting tracks. The solder may be in the form of a strip of material linking the ends of the two tracts such that, on overheating, the solder melts, withdraws onto the terminal ends of the tracks, and breaks the circuit.
Again, installation of such links can be awkward while the nature of the link can be such as to impose a degree of unreliability on the precise temperature at which the circuit is brokenxe2x80x94under certain circumstances during the fusing process such links have been known to drop off the associated substrate.
Alternatively, the solder may comprise a first solder material deposited on the terminal ends of the two tracks, and a second solder material deposited on the first solder material and filling the gap between the track ends. The two solder materials melt at different temperatures, and are chosen dependent on circumstances. In practice, the second solder material melts at the desired actuation temperature of the fuse, and, on melting flows onto the first solder material on the terminal ends to create a gap and break the circuit.
The manufacture of such a fuse is clearly a relatively complex procedure, while the cost is significant in view of the two separate solder materials used.
In all the above-mentioned solder fuses, breaking the circuit is dependent upon on melting of the solder and subsequent separation of the melted solder by flowing onto the spaced-apart conductive terminal ends of the tracks, thereby to create a gap between the terminal ends.
It would be desirable to be able to provide a heat actuated solder fuse for thick film circuits that is more easily and more cheaply produced than heretofore, and which is less reliant upon molten flow than current arrangements.
According to one aspect of the present invention there is provided a thick or thin film circuit comprising an electrically insulating substrate having deposited thereon an electrically conductive track the material of which is capable of being leached by molten solder, and, deposited on said track, a volume of solder having a predetermined melting temperature, the arrangement being such that, on melting of the solder at said predetermined temperature, the molten solder leaches the material of the track, creates a gap therein, and thereby breaks an electrical circuit through said track.
It will be appreciated that such a fuse is readily manufactured, it merely being necessary to deposit the solder in the required position on the substrate, which deposition step can be integrated into the screen printing process associated with formation of the thick film circuit itself.
In a preferred embodiment of the invention, the material of the electrically conductive track is silver or a silver-based alloy, while the solder contains tin.
In a preferred embodiment of the invention, the conductive track provides a resistor of a fan motor controller so the fuse is incorporated into the resistor with a minimum of operations.
The electrically conductive track may be continuous with the volume of solder deposited on, to cover a length of, the continuous track.
Alternatively the track may comprise a pair of spaced-apart terminals with the volume of solder deposited on said terminals and extending across the gap between the terminals.
According to a second aspect of the invention, there is provided a method of manufacturing a thick film circuit as defined above, the method comprising the steps of
providing an electrically insulating substrate;
depositing on said substrate an electrically conductive track the material of which is capable of being leached by molten solder;
depositing onto the substrate and onto all of the electrically conductive track except that part thereof to be covered by the solder a solder resist, and
depositing on said part of the electrically conductive track a volume of solder having a predetermined melting temperature whereby, on melting of the solder at said predetermined temperature, the molten solder leaches the material of the electrically conductive track to create a gap therein thereby to break an electrical circuit through the track.
It is an object of this invention to provide an electrical component having an improved thermal fuse.
A further object of this invention is to provide a method of manufacturing an improved thermal fuse on an insulating substrate.